1. Field of the Invention
The invention relates to an optical waveguide, more particularly, to a quasi-phase matched optical waveguide for preventing a back conversion process occurring during the generation of a conversion wave.
2. Description of the Related Art
Quasi-phase matched (QPM) waveguide devices such as periodically poled lithium niobate waveguides have been applied in optical sampling systems and all-optical logic processes. Such optical sampling systems and all-optical logic processes are based on a specific cascaded quadratic nonlinear process χ2:χ2 in a QPM waveguide to induce cascaded second-harmonic generation (SHG) and difference-frequency generation (DFG). In the cascaded SHG+DFG process, a launched pump wave (P wave) generates its second harmonic wave (PP wave) via a SHG process, and then the second-harmonic wave immediately interacts with another launched signal wave (S wave) to generate a conversion wave (C wave) via a DFG process. The conversion wave also interacts with the second harmonic wave to generate the signal wave via a DFG process. When the phase difference between the conversion wave and its coupling component is out of phase, the coupling component destructively interferes with the conversion wave and reduces the energy of the conversion wave. The process by which pulse energy of the conversion wave declines is called back-conversion which, apparently, reduces the conversion efficiency of the conversion wave of the cascaded SHG+DFG process. In a back-conversion process, the conversion wave interacts with the signal wave to generate the second-harmonic wave by sum-frequency generation. Therefore, both of the energies of the conversion wave and the signal wave decline.
The factors affecting such back conversion have been investigated which include chromatic dispersion, pulse duration and peak power. When the peak power is low, chromatic dispersion strongly affects the back conversion phenomenon. If an input power is high, the magnitude of the power will dominate the back conversion effect.
In Shih-Chiang Lin et al. “Enhanced Cascaded SHG+DFG Process of Femtosecond Pulses Using QPM Waveguide,” IEEE J. Lightwave Tech., 26, 3090-3098, (2008), a chirp QPM waveguide has been proposed to reduce the back conversion effect and enhance the cascaded SHG+DFG process for a lower power case. But such a chirp QPM waveguide is not suitable for high power situations.
Since the back conversion is caused by the destructive interference between the conversion wave and a component resulting from the coupling of the second harmonic wave with the signal wave, removal of such a signal or second harmonic wave component may help to break the mechanism of back conversion. However, as the second harmonic wave is important and cannot be removed, it is only the signal wave that can be removed to prevent back conversion.